All of our modern submarine diesel engines are of the 2-stroke cycle type. | 6 INTAKE AND EXHAUST SYSTEMS A. GENERAL 6A1. Intake systems. All of our modern submarine diesel engines are of the 2-stroke cycle type. The purpose of the intake systems in these engines is to force out the exhaust gases of combustion as effectively as possible and to recharge the cylinder with fresh air in order to support combustion for the next succeeding cycle. The supply of air must be in excess of that required to just support combustion since the fuel is thoroughly mixed with only part of the air compressed within the cylinder. The ratio of air to fuel in most diesel engines is approximately 20 to 1 at full load. 6A2. Scavenging. The term scavenging is used to describe the process of ridding the cylinder of burned exhaust gases during the latter part of the expansion stroke and the early part of the compression stroke of the 2-stroke cycle engine. Scavenging is accomplished by admitting fresh air under a pressure of about 1 to 5 psi into the cylinder while the exhaust valves or ports are open. This pressure usually is developed by means of a scavenging air blower. These blowers are driven from the engines themselves and generally are of the lobed rotor type the rotors revolving together in closely fitting housings. The process of scavenging must be carried out in an extremely short period of time depending upon the speed of the engine. The burned gases must be blown out of the cylinder and a fresh charge of air admitted during the time that the ports or valves are open. For example in an engine making 750 rpm with the exhaust ports open for 140 degrees of crank angle the elapsed time the ports are open each revolution is only 140 360 x 60 750 or approximately 1 32 of a second. The scavenging air must be so directed as to remove the burned gases from the remote parts of the cylinder. The These methods are illustrated in Figures 6-1 to 6-4. In port direct scavenging the exhaust ports are on one side of the cylinder and the scavenging ports on the other. In .
